1. Field of the Invention
The present invention relates to a motion control apparatus for a vehicle, in particular, to a motion control apparatus that executes vehicle stabilization control by imparting braking force to at least one wheel of a vehicle in order to generate a yawing moment in a direction opposite the vehicle's yawing direction, when the vehicle is in a state of over-steering or when the vehicle is experiencing spin tendency.
2. Description of the Related Art
Conventionally, vehicle motion control apparatuses are known that execute the above-described vehicle stabilization control when a vehicle is in a state of over-steering. A conventional motion control apparatus determines that the vehicle is in a state of over-steering when, for example, the difference (hereinafter referred to as “yaw rate deviation”) between the actual yaw rate of the vehicle obtained from a yaw rate sensor (hereafter referred to as “actual yaw rate”) and the vehicle yaw rate calculated from the steering angle, vehicle body speed, vehicle specifications, and the like (hereafter referred to as “steering angle yaw rate”) exceeds a predetermined threshold value. The apparatus then executes the aforementioned vehicle stabilization control (i.e., over-steering suppression control) that imparts a predetermined braking force, by means of brake hydraulic pressure, to the wheel on the outer side of a turning locus. Further, another motion control apparatus recited in Japanese Patent Application Laid-Open (kokai) No. 2003-182556 is also known.
Nonetheless, in cases where, for example, a vehicle is traveling on a low-μ road surface whose friction coefficient is relatively low, and the driver performs an excessively large steering operation in the turning direction, the vehicle first goes into a state of under-steering. After that, the rear wheels slip towards the outer side of the turning locus, which is caused by the generation of a yawing moment in the turning direction of the vehicle, and in some cases, a relatively slow spin tendency occurs in the vehicle. As a result, the vehicle shifts from the state of under-steering to a state of over-steering.
Moreover, when the driver releases the accelerator pedal or steps on the brake pedal in a state where the centrifugal force acting on the vehicle in the middle of turning travel is great to the extent that it is substantially equal to the greatest cornering force that can be generated by the tires (hereafter referred to as “limit turning state”), the load acting on the wheels of the front side of the vehicle increases. Due to this, the load applied to the wheels of the rear side of the vehicle decreases, and the greatest cornering force that can be generated by the rear wheels lowers. As a result, in some cases, the rear wheels slip towards the outer side of the turning locus due to centrifugal force, so that a relatively slow spin tendency occurs in the vehicle. As a result, the vehicle goes into a state of over-steering.
When such spin tendency occurs in the vehicle, in order to maintain the stability of the vehicle, it is preferable to detect the generation of spin tendency, and early initiate and execute the above-described vehicle stabilization control. Nonetheless, in cases where a relatively slow spin tendency is generated, the increasing speed of the above-described yaw rate deviation is typically small. As a result, the yaw rate deviation does not exceed the aforementioned threshold value at an early stage after generation of such spin tendency. Accordingly, there has been a problem in that the vehicle stabilization control cannot be initiated early enough.
In other words, if the generation of relatively slow spin tendency as described above can be detected with certainty, vehicle stabilization control can be initiated at an early stage. For this reason, there is a need for detecting with certainty the generation of spin tendency, in response to which the vehicle stabilization control (over-steering suppression control) must be started at an early stage.